Currently, there is known a radiation image detector, which is a so-called flat panel detector (FPD), as a device to obtain radiation images for medical purposes, the radiation image detector in which solid state image sensors are two-dimensionally arranged. As such a radiation image detector, there are known a radiation image detector using a direct method by which radiation energy is directly converted into an electric charge by using photo conductive material such as a-Se (amorphous selenium) as radiation detection elements, and the electric charge is read as electric signals of their respective pixels by using switching elements for reading signals such as TFTs (Thin Film Transistors) being two-dimensionally arranged, and a radiation image detector using an indirect method by which radiation energy is converted into light by using scintillators or the like, and the light is converted into an electric charge by using photoelectric conversion elements such as photo diodes being two-dimensionally arranged, and then the electric charge is read as electric signals by TFTs or the like.
No matter which method is used for a radiation image detector, it is known that radiographed image data needs to be corrected by gain correction, offset correction, and/or the like, the radiographed image data which is obtained by detecting a radiation by the radiation image detector, the radiation passing through a subject.
Gain correction values and offset correction values are necessary to perform the gain correction and the offset correction, respectively. Since these correction values fluctuate over time, the correction values are updated by regularly performing calibration on a radiation image detector, in general.
In particular, it is known that the fluctuation cycle of offset correction values is shorter than the fluctuation cycle of gain correction values (namely, offset correction value easily fluctuate), and that offset correction values largely change according to a temperature change and the like.
Hence, in order to understand the change of the characteristics of offset correction values over time, offset calibration which appropriately updates offset correction values is performed, the offset correction values calculated based on dark reading values which are obtained by regularly performing so-called dark reading by which electric signals are read without irradiating a radiation image detector.
When dark reading values are obtained, various electric noise, namely, dark current noise of a photo diode, TFT transient noise, TFT thermal noise, TFT leak noise, thermal noise caused by a parasitic capacitance of a data line which reads an electric charge from a TFT, amplifier noise inside a reading circuit, quantization noise caused by A/D conversion, and the like, are superposed on signals thereof, and influences the dark reading values, accordingly.
Therefore, when offset correction values are calculated, in many cases, in order to reduce the influence of the noise which is electrically superposed on signals read by the dark reading, the dark reading is performed on each of radiation detection elements multiple times. Then, the average value of dark reading values obtained thereby is obtained with regard to each of the respective radiation detection elements so that the obtained average values are used as the offset correction values. (Refer to Patent Documents 1 to 3, for example.)
Currently, there is developed a portable radiation image detector which has a built-in battery as an internal power supply section so as to be driven without a cable.
In addition, there are proposed a portable (cassette type) radiation image detector which has a built-in battery so as to be capable of radiography with a high degree of freedom by not using a cable, such as portable radiography at a bedside and the like, and also be capable of keeping radiography with no worries about the remaining charged amount of a battery by using a cable in a radiography room and the like so that power is supplied from an external power source (Patent Document 4, for example), and a portable radiation image detector to which power is supplied from a built-in battery when the portable radiation image detector is used by itself, and to which power is supplied from a Bucky device when the portable radiation image detector is loaded into the Bucky device (Patent Document 5, for example).
When a battery is charged by supplying power from an external power source, the battery is generally charged at a high speed by a large amount of power being supplied at a time from the external power source, and hence, a charging time of the battery is shortened.    Patent Document 1: U.S. Pat. No. 5,452,338    Patent Document 2: U.S. Pat. No. 6,222,901    Patent Document 3: U.S. Pat. No. 7,041,955    Patent Document 4: Japanese Patent Application Laid-open Publication No. 2003-172783    Patent Document 5: Japanese Patent Application Laid-open Publication No. 2000-347330